Mass spectrometry



Oct. 5, 1948. w, WASHBURN 2,450,462

Mass SPECTROMETRY Filed Nov. 4. 1943 YINVYENTOR. #12040 /I Ill/515201? AT T OFNEKS Patented Oct. 5, 1948 MASS SPECTROMETRY Harold W. Washburn,Pasadena, Calif., assignor to Consolidated Engineering Corporation,Pasadena, Calif., a corporation of California Application November 4,1943, Serial No. 508,923

This invention is concerned with mass spectrometry and providesimprovements that are particularly useful in the analysi of mixturescontaining organic compounds such for example as hydrocarbons, whichcrack under the conditions to which they are subjected in the ionizationchamber of 1a massspectrometer.

This application is a continuation-in-part of my co-pending applicationSerial No. 451,664, filed July 20, 1942, now Patent No. 2,387,785,October 30, 1945.

4 Claims. (Cl. 25041.9)

I chamber.

Basically a mass spectrometer-is an apparatus for producing and sortingions, and comprises an ionization chamber, an analyzer and a collector.Material to be analyzed, for example a gas mixture, is introduced intothe ionization chamber from a sample chamber at relatively low pressure. In the ionization chamber, the molecules of the mixture, or someof them, are bombarded With ionizing particles, say electrons, andconverted into ions. With the aid of one or more electric fields,- theions are propelled out of the ionization chamber into the analyzer,where they are separated into a plurality of ion beams according totheir mass-to-charge ratio, i. e. their specific mass. The several ionbeams are discharged separately at the collector, and the current givenup in each case is measured with a galvanometer or the like to produce amass spectrum.

A thermionic emitting element, for example a heated filament, isconvenient mechanism for generating an electron beam with which tobombard the molecules in the ionization chamber, but the presence of theheated filament in the ionization chamber itself is disadvantageous inmany instances, particularly when the mixture to be ionized containscompounds, such as hydrocarbons, which tend to crack. In such cases thepresence of the electron beam source in the ionization chamberintroduces irregularities, errors, and inconsistencies in the spectrum,.so that qualitative and quantitative analysesof the mixture aredifiicult. I have found that the accuracy of the mass spectrum in suchcases is increased if the heated element employed for producing theelectron beam is enclosed in a separate chamber connected to theionization chamber by a conduit (aperture) through which the electronbeam is projected, means being provided for retarding the entry of gasfrom the separate chamber into the ionization chamber and for keepingthe amount of gas so entering the ionization chamber small compared withthe amount of gas entering the ionization chamber from the sample Inthis way, the entry into the ionization chamber of products of thermalcracking formed in the separate chamber is inhibited,and the consistencyand reproducibility of the spectrum is improved, apparently because inthese circumstances and'with proper conditions of temperature andpressure in the ionization chamber, electronic bombardment of themolecules of the mixture results in more regular cracking patterns.Whatever be the explanation, the fact remains that improved results areobtained if the electron beam source (say a filament) is disposed in aseparate chamber which communicates with the ionization chamber througha conduit or aperture that serves as an avenue for the entry of theelectron beam to the ionization chamber, the pressure in the separatechamber being low (say of the order of 10 mm. Hg) and preferably lowerthan the pressure in the ionization chamber.

The proper relationships of pressure in the filament chamber (electronbeam source) and ionization chamber can-be obtained by (a) mak ing thepumping speed between the filament chamber and the evacuating apparatushigh in comparison with the pumping speed between filament chamber andthe ionization chamber and by (12) making the conduit between the twochambers very small.

Expressed in another Way PzPy Pz+Px should be large in comparison withWhere Py is the pumping speed of the conduit directly connecting thefilament (electron emitting) chamber with the ionization chamber;

P2 is the pumping speed of the conduit directly connecting the filamentchamber with the region adjoining the ionization chamber (Fig. 1)

Pat is the pumping speed of the conduit directly connecting theionization chamber to the region adjoining that chamber, and

Pw i the pumping speed from the region adjoining the ionization chamberto the pump.

A further beneficial result can be obtained by placing the filament at apoint in the filament chamber that is remotefrom the conduit throughwhich the electron beam (originating at the filament) enters theionization chamber, thereby reducing the direct evaporation of crackedgas from the filament.

I believe that the benefits accruing to the practice of my inventionwhen employing a filament (metallic or oxide coated) a an electronsource, are attributable at least in part to the fact that the emittingcharacteristics of the surface of the filament are aifected by thepresence of gas and that the presence of gas may have both long term andtemporary effects. It is desirable to minimize the first and essentialto minimize the secnd in order to obtain reproducibility, and themaintenance of a low pressure in the region of the filament is a step inthis direction.

I have observed that in some cases a gas has a differential effect uponthe emitting characteristics of a metallic filament surface, 1. e.different portions of the surface are affected clifierently, the resultbeing that the change in density of electrons at various points acrossthe beam is not proportional to electron density at the respectivepoints. The positive ions which are recorded are selected from only asmall portion of the beam and in the circumstances indicated, tend to bean unrepresentative sample. The practice of the invention tends toimprove the representativeness of the sample by reducing thedifferential emitting effect at the filament.

Experiment has shown that the amount of gas getting back into theionization chamber after having been in contact with the filament shouldnot exceed 1% of the gas sample, i. e. that introduced into theionization chamber from a sample bottle or other outside source.Moreover, the amount of gas which is permitted to return to theionization chamber after contact with the filament shields or other hotmetal in the filament chamber should be less than 2% of the gas sample.

These and other features of my invention will be more thoroughlyunderstood in the light of the following detailed description, taken inconjunction with the accompanying drawings in which:

Fig. 1 is a fragmentary diagram of a mass spectrometer adapted to thepractice of my invention;

Figs. 2 and 3 are longitudinal sections, taken at right angles to eachother, through a preferred form of spectrometer head constructed inaccordance with the invention.

Referring now to Fig. 1, it will be observed that the mass spectrometercomprises an envelope Ii) of glass, or the like, adapted to bemaintained during operation at low pressure by evacuation of gas througha conduit I I b means of vacuum pumps (not shown). Within the envelopethere is disposed an ionization chamber I2 connected at one end to aconduit I3 through which a sample of gas to be analyzed may be admitted.The other or outlet end of the ionization chamber is connected to ananalyzer tube I4 through slits S1, S2, respectively, in two propellingelectrodes I5, I6 disposed in series. The analyzer tube issemi-circular, as is the envelope, and has an exit slit IT at itsoutlet. An ion collector or target I8 is disposed within the envelopeimmediately adjacent the exit slit from the analyzer tube. The collectoris connected to appropriate amplifier and recording means IS.

A pusher electrode 28 is disposed within the ionization chamber in linewith the slits S1, S2. An electron gun 2|, for example an electricallyheated filament and an appropriate electrode, is disposed in a chamber22 which communicates with the ionization chamber at one side thereofthrough an aperture S3. A beam or stream 23 of electrons is projected bythe electron gun through the aperture S3 into the ionization chamher atright angles to a line connecting the pusher electrode 20 and the slitsS1, S2. The chamber 22 within which the electron gun is mountedcommunicates with the envelope through a slit S4.

The analyzer tube may be evacuated in part through the ion exit slit Has well as through auxiliary openings 25, 25A, in the wall of theanalyzer.

In the operation of the apparatus illustrated, a gas sample to beanalyzed is introduced into the ionization chamber through the inletconduit and the molecules of this gas sample are there bombarded byelectrons of the beam. In this way, molecules of the gas sample becomeionized and the resulting ions are expelled from the ionization chamberinto the analyzer chamber under the influence of a pusher potentialmaintained between the electrode 20 and the electrode I5 and anadditiona1 accelerating potential maintained between the electrode I5and the electrode I6. In this way, a heterogeneous beam of ions ispropelled from the ionization chamber into the analyzer tube. A stronmagnetic field produced by an electromagnet (not shown) causes theheterogeneous ion beam entering the analyzer tube to curve and to beseparated into a plurality of diverging ion beams according to thespecific mass of the ions.

By varying the potentials which propel the ions into and through theanalyzer tube or by varying the strength of the magnetic field or both,the radii of curvature of the several ion beams in the tube may bealtered, so that the diverging beams are caused to sweep successivelyover the exit slit I? and impinge on the ion collector I8. The currentsthus collected from the diverging ion beams are amplified and separatelyrecorded and constitute the mass spectrum of the material undergoinganalysis.

As indicated hereinbefore, the thermionic electron emitting element (i.e. the electron gun 2 I) tends to bring about pyrolysis of hydrocarbonsand other organic chemicals which come in contact with the filament.Accordingly, pursuant to my invention, the thermionic electron source isenclosed in a separate chamber 22 connected to the ionization chamberthrough the conduit S3. Th electron beam is projected through this 0011duit and means are provided to inhibit the passage of products ofthermal cracking formed in the neighborhood of the electron emittingelement into the ionization region by maintaining the pressure in theionization chamber greater than that in the electron emission chamber.

Since in the instant case the entire apparatus is evacuated by a commonvacuum pump through the conduit I I, the maintenance of a higherpressure in the ionization chamber than in the electron emission chamberrequires that the gas flow resistance of the passages for evacuating theelectron emission chamber shall be less than the gas flow resistance ofthe passages for evacuating the ionization chamber. With thisrelationship of gas flow resistance, a relatively high gas pres sure maybe built up in the ionization chamber. This permits the attainment ofhigh sensitivity with the instrument. At the same time irregularities inmass spectra due to products cracked in the neighborhood of the electronemitting means are, for practical purposes, eliminated.

A preferr d form of instrument head, including anfionization chamber andan electron emittin chamber, is illustrated inFigs. 2 :and3, which arelongitudinal sections through the head taken at right. angles to .eachother. As shown in these figures, the apparatus comprises a cylindricalhead 30 attached to the end of the analyzer tube [4 mounted within theenvelope I, which is adapted to be evacuated through the pumping line H.The end of the head opposite the analyzer tube is connected to the gasinlet conduit 13;

During operation, the envelope (and the head and analyzer tube which itencloses') are maintained at'relatively low pressure, the pressure inthe ionization chamber being greater than that in the filament chamberor'analyzer tube. By way'of example, the analyzer tube and the filament'chamber may be maintained at 10* mm. Hg or less While the pressure inthe ionization chamber is of the order of 10- or 10- mm. Hg.

To consider the head in greater detail, it will be observed that itcomprises a block 35 which is a thickwalled cylinder having a circularbore that-defines an ionization chamber 36. A quartz disc 31 issecuredto the inlet end of the block. A pair of conductive pushersegments 38, 39 project through the quartz disc into the bore. A quartzplate 40 is clamped between the two pusher segments and serves toinsulate them from each other and at the same time to separate the spacewhich they enclose into a pair of parallel passages or channels 4 I, 42.

A pair of jaws 46, 41 form the other end of the ionization chamber andare fastened to the other end of the block. They have knife edges whichare spaced slightly from each other to define a first slit S1 thatbisects the cylindrical head and is parallel to a gap between the endsP1, P2 of the pusher segments. A Pyrex spacer ring 50 is disposedimmediately adjacent the first pair of jaws and carries a second pair ofjaws 5|, 52 matching the first pair with a slit S2 therebetween that isparallel to the slit S1. The second pair of jaws is separated fromtheanalyzer tube by a conductive ring 54 and a head mounting flange 55which is fastened to the end of the anlyzer tube, and-is electricallyconnected to the second pair of jaws, to the analyzer tube and toground.

The pusher segments are secured to the block by means of a pusher clamp51, which covers a Pyrexpusher locking ring 58 and is screwed to theblock through the quartz disc.

The block, the first pair of jaws, the Pyrex insulating, ring, thesecond pair of jaws, the conductive spacer and the head mounting flangeare held together with quartz links 60. Each link has an eye that ispositioned over a link stud screw Bl that projects from the side of theblock. The other end ofthe ring carries a second eye which is secured tothe head mounting flange by means of spring link clips52.

One side of the block is cutaway to forma space 65 within which anelectron gun 66 is mounted. The space in which the gun is mounted isenclosed on one end by the quartz disc, on the inside by the wall of theblock, and on the other end and outside by an L-shaped shield 61 ofmetal. This shield is spaced from the quartz block by an evacuation gapor slot 68.

The electron gun comprises a filament '10 adapted to be heated byelectric current and partly enclosed by a box-shaped metallic shield H.At the level of the filament there is a bore 1'2 through the wall of theblock into the ioniza This bore carries an apertured: barrel or insert13. An electron propelling election chamber.

' block.

In line with the barrel of the electron gun and on the opposite side ofthe block is a bore 801 through which electrons of the beam pass to anelectron catcher 8| that is disposed within a second bore 82 at rightangles to the bore '80.

As indicated hereinbefore, the apparatus is so constructedthat the gapbetween the pusher segments, the axis of the electron beam, and the axesof the slits S1 and S2 are all parallel to each other and also parallelto lines of force of a magnetic field in which the assembly is disposed.

Considered from an electrical standpoint, the block and the first pairof jaws that define the slit Sr represent a first-ion acceleratingelectrode. The jaws forming thesecond slit comprise a second ionaccelerating electrode, whereas the two pusher segments are pusherelectrodes. An electrical potential is impressed between the pusherelectrodes and the first accelerating electrode,

and the second electrical potential is impressed between thefirst andsecond accelerating electrodes;

The voltages of the filament, of the electron accelerating electrode, ofthe pusher electrodes,

and of the first ion accelerating electrode may be controlledindependently. In normal operation when analyzing for positive ions, allof the above are maintained at high potentials with respect to thesecond-ion accelerating electrode which, as

indicated hereinbefore, is at ground.

A thermo-couple 90 is attached to the side block for measuring the blocktemperature.

A gas mixture to be analyzed is introduced into the ionization chamberthrough the channels between the pusher segments. Within the ionizationchamber the molecules of' gas are bombarded by the electron beam andsome of them are ionized. The resulting ions are pushed through the slitSi by the pusher potential and further accelerated by additionalpotential through the slit S2 and thence into the analyzer tube. Thus, aheterogeneous'ion beam is propelled into the analyzer tube and is thereseparated into a plurality of homogeneous ion beams which are collectedseparately to form the mass spectrum.

Some of the gas introduced into the ionization chamber is not ionized.This gas for the most partflows through the slit S1 and thence throughports (not shown) in the jaws 5|, 52 into the analyzer tube. From theanalyzer tube this gas escapes into the envelope through ports 92 in thewall of the analyzer tube. From this point the gas is evacuated throughthe pumping line H.

Some of the gas flows through the barrel of the electron gun into theregion of the filament.

From this point the gas (either unchanged or in a cracked condition) isin large part withdrawn through the evacuation gap or slot 68 into theenvelope and thence through the pumping line.

There are cracks in the box-shaped filament shield which permit gases toflow from the region of the filament through the exit slot 68.

As indicated previously, it is desirable from the standpoint of accuracyand reproducibility of the mass spectrometer to make sure that thepassage of particles other than electrons into the ionization chamberthrough the electron gun barrel is inhibited by making the pressure inthe space enclosing the electron gun less than the pressure in theionization chamber itself.

The pressure differential between the ionization chamber and theelectron gun chamber is maintained in the apparatus of Figs. 2 and 3 byassuring that the avenue of escape of gases from the electron gunchamber into the pumping line, offers less resistance than does theavenue of the escape of gases from the ionization chamber, this beingaccomplished by properly proportioning the pumping speeds or resistancesalong the respective avenues, as described hereinbefore.

I claim:

1. In a mass spectrometer having an ionization chamber, a samplechamber, means for introducing a material to be analyzed into theionization chamber from the sample chamber, means for shooting anelectron beam against molecules of the material in the ionizationchamber, an analyzer connected to the ionization chamber through arestricted passage, and means for expelling ions formed from thematerial from the ionization chamber into the analyzer, the combinationwhich comprises an electron producer chamber, means disposed in thatchamber for producing an electron beam, a conduit connected between theelectron producer chamber and the ionization chamber for admitting thebeam to the latter, said conduit acting to restrict the flow of gas fromthe ionization chamber to the electron producer chamber, and a pump forevacuating the electron producer chamber, the ionization chamher and theanalyzer, said pump being connected to the electron producer chamber andthe analyzer so that gas is drawn from the ionization chamber into boththe electron producer chamber and the analyzer.

2. In a mass spectrometer having an ionization chamber, means forintroducing a material to be analyzed into the ionization chamber, meansfor shooting an electron beam against molecules of the material in theionization chamber, an analyzer connected to the ionization chamber, andmeans for expelling ions formed from the material from the ionizationchamber into the analyzer, the combination which comprises an electronchamber, means disposed in that chamber for producing an electron beam,a conduit connected between the electron producer chamber and theionization chamber for admit-ting the electron beam to the latter, meansfor directly evacuating the electron producer chamber and the analyzer,and a restriction in the conduit between the electron producer chamberand the ionization chamber and a restriction in the means for expellingions into the analyzer for retarding the flow of gas from the ionizationchamber to the electron producer chamber and the analyzer, thearrangement being such that the ionization chamber is evacuated throughthe electron producer chamber and the analyzer.

3. In a mass spectrometer having an ionization chamber, means forintroducing a material to be analyzed into the ionization chamber, meansfor shooting an electron beam against molecules of the material in theionization chamber, an analyzer connected to the ionization chamber, andmeans for expelling ion-s formed from the material from the ionizationchamber into the analyzer, the combination which comprises an electronproducer chamber, means disposed in that chamber for producing anelectron beam, a conduit connected between the electron producer chamberand the ionization chamber for admitting the electron beam to thelatter, and a pump connected to evacuate both the electron chamber andthe analyzer, a second conduit between the ionization chamber and theanalyzer for allowing ions to pass in to the analyzer, both of saidconduits having a restricted opening so that when the pump is operating,the evacuation of the electron producer chamber and the analyzer drawsgas from the ionization chamber and causes it to be at a high-erpressure than the analyzer and the electron producer chamber.

4. In a mass spectrometer having an ionization chamber, means foradmitting a material to be analyzed into the ionization chamber, ananalyzer connected to the ionization chamber, and means for expellingions formed from the material from the ionization chamber into theanalyzer, the combination which comprises an electron source, arestricted passage disposed between the electron source and theionization chamber and through which a beam of electrons from the sourceis shot into the chamber, a second restricted passage between theionization. chamber and the analyzer through which the ions areexpelled, a pump connected to the analyzer for evacuating it and alsoconnected to the region of the electron source, so that gas is drawnfrom the ionization chamber simultaneously through both restrictedpassages. HAROLD W. XNASHBURN.

REFERENCES CITED The following references are of record in the file ofthis patent:

Technical publication A Mass Spectrum Analysis of the Products'ofIonization by Electron Impact in Nitrogen, Acetylene, Nitric Oxide,Cyanogen and Carbon Monoxide, by John 'I'. Tate, 'P. T. Smith and A. L.Vaughan. Physical Review, Vol. 48, Sept. 15, 1935, pages 525-531.

Technical publication A Mass Spectrometer for Routine Isotope AbundanceMeasurements, by Alfred 0. Nier, Review of Scientific Instruments, vol.11, July 1940, pages 212-216.

